A number of the applications for silicon carbide depend upon the friction and wear (tribological) characteristics of the material, in addition to its outstanding thermal, chemical and mechanical properties. These applications include, for example, mechanical seal faces, valve lifters and nozzles. Mechanical seal applications are especially demanding in that the seal members are subjected to both a large compressive force (P) normal to the surface of the face and a sliding velocity (V) across the surface of the face. In evaluating the performance of mechanical seal members, the product (PV) under which the seal can operate is an important parameter.
Certain materials, such as boron nitride, graphite, fluorocarbons and silicones, are self-lubricating, but silicon carbide itself is not one of these. Consequently, mechanical seals which employ silicon carbide as one or both of the seal members may exhibit a squeaking sound when the faces are engaged, or excessive force may be required to initiate separation of the faces and opening of the seal. These behaviors are not desirable, and attempts have been made to introduce lubrication at the seal face surfaces to eliminate these problems. The very liquid upon which the seal operates can be called upon to perform this lubricant role.
The use of various forms of silicon carbide in mechanical seal applications has been described by Lashway in Lubrication Engineering, 40, 356 (1984). Among the forms of silicon carbide tested by Lashway was a sintered silicon carbide containing "controlled porosity"; the manner in which it was produced was not disclosed. The pores in this silicon carbide, comprising 8-10 volume percent ("vol %" hereinafter) of the body, were said to be closed and small, i.e., 20 microns. For purposes of this application, vol % porosity=100-100.times. [observed density/theoretical density]. Lashway attributed the superior mechanical seal face performance of his porous silicon carbide to its ability to retain a hydrodynamic film of the sealed liquid on the seal face.
Silicon carbide of theoretical density (3.21 g/cm.sup.3) is seldom seen in commercial applications, and the silicon carbide of lesser density which is used generally contains a second, pore phase. These pores may be void or filled with silicon or some other material, depending upon the processing route and desired properties. Porosity of a few vol % is not a factor in many silicon carbide applications and is generally ignored. U.S. Pat. Nos. 4,179,299 and 4,312,954 describe silicon carbide of less than theoretical density, and the silicon carbide disclosed therein can be found in many commercial mechanical seal applications. A more recent patent, U.S. Pat. No. 4,525,461, discloses a sintered silicon carbide which also contains graphite. This graphitized silicon carbide, which is said to be self-lubricating and useful in mechanical seal applications, also contains pores. Thus, silicon carbide sintered bodies which contain pores are not per se new.
The presence of pores in silicon carbide can be detrimental to its performance to the extent physical properties of the sintered body affect its performance. In this regard, Seshadri, et al., Ceramic Trans., 2, 215 (1987), reported preparing sintered silicon carbide bodies with a series of porosities in the range 1-7.5 vol % by altering the sintering parameters and carbon additions. The flexural strength of the resultant sintered silicon carbide bodies was reported to be insensitive to the porosity up to a porosity of 7 vol %, but the elastic constants and fracture toughness were affected significantly.
Introducing porosity into a sintered silicon carbide body by variations in the sintering time/temperature profile has not been reproducible enough to enable this technique to be used for making commercial porous material. Hence, such materials do not exhibit the "controlled" porosity referred to hereinafter. Another method that can produce porous sintered silicon carbide, involves reduction in the amount of sintering aids. This method decreases densification, and a porous body with lower density is produced. Control over this process is the major deterrent. Precise control of the raw materials is necessary to make this method dependable. This is not possible with today's materials at reasonable cost, thus making the processing window for this method very small, and hard to predict.
Porous sintered materials can also be produced by adding other materials that thermally degrade and/or shrink, such as cellulose, plant products of many types and shapes/sizes, and inorganic materials such as glass or low temperature ceramics. These materials, may, however, present difficulties in mixing with the fine silicon carbide powder and, further, may interfere with the sintering mechanism.
Consistent with Lashway's 1983 report, it was disclosed in German Offen. DE 3927300, laid open Feb. 2, 1990, that sintered silicon carbide bodies having between 4 vol % and 13 vol % porosity, where the pores averaged between 10 microns and 40 microns in diameter, provided superior mechanical seals. Such porous bodies were prepared by introducing spherical organic polymeric beads into the raw batch from which the green bodies were produced and then heating the green bodies to remove the organic and densify the body. It was observed that, at pore volumes less than 4 vol %, the porosity was insufficient to effect the improvement, while at pore volumes greater than 13 vol %, the strength of the body was adversely affected, and the pores intercommunicated, causing leakage of the seal. Further, if the average pore diameter was less than 10 microns, the lubricating liquid in the pores was not sufficiently available, while at pore diameters greater than 40 microns, carbon seal face wear was accelerated and seal leakage occurred. It was said that the pores must be rounded in order to avoid stress concentration in the sintered body.
Whereas the introduction of porosity into a sintered silicon carbide body can be effected by introducing organic polymer beads into the raw batch, that technique is not without disadvantages. For example, the polymer must be homogeneously dispersed in the green ceramic body and then be removed from the ceramic body if pores are to be created. Presumably this occurs by thermal decomposition of the polymer beads in the sintering step. However, it is difficult for the resultant products of decomposition to leave the pores if they are truly independent; the interior pores probably communicate with other pores closer to the surface of the body. To the extent the decomposition products remain in the sintered body they constitute contaminants which can affect the physical properties of the ceramic body. Furthermore, the somewhat elastic polymer beads can be compressed under the pressure utilized in forming the green ceramic body; release of the pressure with recovery of the beads can introduce microcracks in the surrounding silicon carbide matrix. In addition, the use of polymer beads can add additional manipulative steps and materials expense which may be reflected in the price of the product.